Method for a 20 KSI BOP Stack with shared differential

ABSTRACT

In a blowout preventer stack with two sealing elements which will not individually withstand the desired pressure differential the method of withstanding the desired pressure differential comprising providing a lower sealing element and an upper sealing element, providing a vent port in the bore below the lower sealing element to a relief valve, venting the outlet of the relief valve to the bore between the lower sealing element and the upper sealing element, adjusting the relief valve to limit the pressure allowed below the lower sealing element to a predetermined amount equal to or less than the working pressure of the lower sealing element.

TECHNICAL FIELD

This invention relates to the method of providing a 20,000 p.s.i. blowout preventer stack by using a shared pressure differential on components which cannot individually be rated to 20,000 p.s.i.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

REFERENCE TO A “MICROFICHE APPENDIX”

Not applicable

BACKGROUND OF THE INVENTION

Deepwater offshore drilling requires that a vessel at the surface be connected through a drilling riser and a large blowout preventer stack to the seafloor wellhead. The seafloor wellhead is the structural anchor piece into the seabed and the basic support for the casing strings which are placed in the well bore as long tubular pressure vessels. During the process of drilling the well, the blowout preventer stack on the top of the subsea wellhead provides the second level of pressure control for the well. The first level being provided by the weighted drilling mud within the bore.

During the drilling process, weighted drilling mud circulates down a string of drill pipe to the drilling bit at the bottom of the hole and back up the annular area between the outside diameter of the drill pipe and the inside diameter of the drilled hole or the casing, depending on the depth.

Coming back up above the blowout preventer stack, the drilling mud will continue to travel back outside the drill pipe and inside the drilling riser, which is much large than the casing. The drilling riser has to be large enough to pass the casing strings run into the well, as well as the casing hangers which will suspend the casing strings. The bore in a contemporary riser will be at least twenty inches in diameter. It additionally has to be pressure competent to handle the pressure of the weighed mud, but does not have the same pressure requirement as the blowout preventer stack itself.

As wells are drilled into progressively deeper and deeper formations, the subsurface pressure and therefore the pressure which the blowout preventer stack must be able to withstand becomes greater and greater. This is the same for drilling on the surface of the land and subsea drilling on the surface of the seafloor. Early subsea blowout preventer stacks were of a 5,000 p.s.i. working pressure, and over time these evolved to 10,000 and 15,000 p.s.i. working pressure. As the working pressure of components becomes higher, the pressure holding components naturally become both heavier and taller. Additionally, in the higher pressure situations, redundant components have been added, again adding to the height. The 15,000 blowout preventer stacks have become in the range of 800,000 lbs. and 80 feet tall. This provides enormous complications on the ability to handle the equipment as well as the loadings on the seafloor wellhead. In addition to the direct weight load on the subsea wellheads, side angle loadings from the drilling riser when the surface vessel drifts off the well centerline are an enormous addition to the stresses on both the subsea wellhead and the seafloor formations.

When the blowout preventer stack working pressure is increased to 20,000 p.s.i. some estimates of the load is that it increases from 800,000 to 1,200,000 lbs. The height also increases, but how much is unclear at this time but it will likely approach 100 feet in height.

A second complication is that a 20,000 p.s.i. working pressure requires a 30,000 p.s.i. test pressure. As the actual stresses in material is greater than the bore pressure, the differential between the actual stress level and the yield strength of the material becomes much narrower. Imagine for a 15,000 p.s.i. component the maximum stress is 32,000 p.s.i. at working pressure and 48,000 p.s.i. at the 22,500 p.s.i. required test pressure. If the best reasonably available material has a 75,000 p.s.i. yield strength at that point you are working with a 1.56/1 factor. If you simply increase the working pressure to 20,000 p.s.i. with a 30,000 p.s.i. test pressure, the stress at test pressure goes to 72,000 p.s.i. which has barely a 1.04/1 safety factor. With the complications of stress analysis, even doubling the weight of the components will not get the stress levels back down to a reasonable level.

Another complication is that the annular style blowout preventer which have the ability to seal on anything in the bore have been characteristically pressure limited, with 10,000 p.s.i. being the highest presently achieved working pressure rating. The large mass of rubber in the donut around the pipe simply fails at that point.

This has been a problem especially since the working pressure of blowout preventers have exceeded 10,000 p.s.i. as the 15,000 and 20,000 p.s.i. differentials across the sealing elements has not been sustainable. The standard industry solution to this point is to accept the inability of annular blowout preventers to seal at this high pressure and to solely depend on the capabilities and limitations of ram blowout preventers when the pressure differential exceeded 10,000 p.s.i.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to give the capability of 15,000 and 20,000 p.s.i blowout preventer stacks to be fully related to 15,000 and 20,000 p.s.i. respectively.

A second object of this invention is to vent the pressure on one blowout preventer above its rating to a downstream blowout preventer.

A third object of this invention is to make the pressure differential between two blowout preventer rams in series adjustable.

Another object of this invention is to make the differential seen by two blowout preventers in series equal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of a contemporary deep-water riser system.

FIG. 2 is a perspective view of a blowout preventer stack utilizing the features of this invention.

FIG. 3 is a perspective view of a subsea wellhead housing which the blowout preventer stack of this invention would land on.

FIG. 4 is a perspective view of the lower portion of the blowout preventer stack of FIG. 2 , generally called the lower BOP stack.

FIG. 5 is a perspective view of the upper portion of the blowout preventer stack of FIG. 2 , generally called the lower marine riser package or LMRP.

FIG. 6 is a perspective view of a section of the drilling riser which will be used to lower the blowout preventer stack.

FIG. 7 is a view of the blowout preventer stack of FIG. 2 , taken along lines “7-7.

FIG. 8 is a view of the blowout preventer stack of FIG. 2 , taken along lines “8-8.

FIG. 9 is a top view of FIG. 8 .

FIG. 10 is cross section view of a double blowout preventer.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 , a view of a system 20 which might use the present invention is shown. It shows a floating vessel 22 on a body of water 24 and having a derrick 26. Drill pipe 28, drilling mud system 30, control reel 32, and control cable 34 are shown. A riser system 40 including a flex joint 42 is shown. During drilling the drilling mud circulated from the drilling mud system 30, up the standpipe 44, down the drill pipe 28, through the drill bit 46, back up through the casing strings 48 and 50, through the blowout preventer stack 60, up thru the riser system 40, and out the bell nipple at 62 back into the mud system 30.

Blowout preventer stack 60 is landed on a subsea wellhead system 64 landed on the seafloor 66. The blowout preventer stack 60 includes pressurized accumulators 68, kill valves 70, choke valves 72, choke and kill lines 74, choke and kill connectors 76, choke and kill flex means 78, and control pods 80.

Referring now to FIG. 2 , the seafloor drilling system 100 comprises a lower blowout preventer stack 102, a lower marine riser package 104, a drilling riser joint 106, and control cables 108.

Referring now to FIG. 3 , a subsea wellhead is shown which the seafloor drilling system lands on. It is the unseen upper portion of the subsea wellhead system 64 shown in FIG. 1 .

Referring now to FIG. 4 , the lower blowout preventer stack 102 comprises a lower structural section 120, vertical support bottle 122, and upper structural section 124, accumulators 126, choke and kill valves 128, blowout preventers 130 and an upper mandrel 132 which will be the connection point for the lower marine riser package.

Referring now to FIG. 5 the lower marine riser package 104 is shown comprising a lower marine riser package structure 140, an interface 142 for a remotely controlled vehicle (ROV), annular blowout preventers 146, choke and kill flex loops 148, a flexible passageway 150, a riser connector 152, and an upper half of a riser connector 154.

Referring now to FIG. 6 , a drilling riser joint 106 is shown having a lower half of a riser connector 160, a upper half of a riser connector 154, and buoyancy sections 162.

Referring now to FIG. 7 , is a view of seafloor drilling system 100 taken along lines “7-7” of FIG. 1 showing wellhead connector 170, lower marine riser connector 172, a man 174 for size perspective, and choke and kill valves 176.

Referring now to FIG. 8 , is a view of seafloor drilling system 100 taken along lines “8-8” of FIG. 1 .

Referring now to FIG. 9 , is a top view of seafloor drilling system 100.

Referring now to FIG. 10 , blowout preventer 200 is shown having a lower thick-walled body section 202 capable of withstanding 20,000 p.s.i., an upper thinner walled body section 204 capable of handling 10,000 p.s.i., a central bore 206, a lower sealing element 208, and upper sealing element 210, a port 212 from the central bore below the lower ram 208 to a relief valve 214 set to a 10,000 p.s.i. pressure differential, and a port 216 to the central bore 218 between the lower sealing element 208 and the upper sealing element 210. When both the lower sealing element 208 and the upper sealing element 210 are closed, any pressure up to 10,000 p.s.i. will be sealed by the lower sealing element 208 and the upper sealing element 210 will not see any pressure differential. As the pressure differential exceeds 10,000 p.s.i., the relief valve 214 will begin to relieve the excess pressure and the upper sealing element 210 will seal all excess pressures above the 10,000 p.s.i. The sealing elements can be of a variety of types, with the most likely being such as the annular sealing elements shown in U.S. Pat. No. 3,572,627.

As 10,000 p.s.i. differential across the lower sealing element 208 puts very high stresses in the resilient materials, it may be preferable to distribute the pressure differential differently that a full 10,000 p.s.i. across the lower sealing element before beginning to load the upper sealing element. Relief valve 214 can be remotely controlled as is illustrated by line 220 going to controller 222 in a different pattern such as the differential being evenly divided between the sealing elements such that at a 10,000 p.s.i. total differential each of the sealing elements withstand is the stress of a 5,000 p.s.i. differential.

The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below. 

That which is claimed is:
 1. In a blowout preventer stack with a bore and with two sealing elements which will not individually withstand the desired pressure differential, a method of withstanding the desired pressure differential comprising providing a lower sealing element and an upper sealing element, providing a vent port in the bore below the lower sealing element to a relief valve, venting the outlet of the relief valve to the bore between the lower sealing element and the upper sealing element, adjusting the relief valve to limit the pressure allowed below the lower sealing element to a predetermined amount equal to or less than the working pressure of the lower sealing element.
 2. The method of claim 1, further comprising the upper sealing element and the lower sealing element are in the same blowout preventer body.
 3. The method of claim 2, further comprising the upper sealing element and the lower sealing element are in the same blowout preventer body and the portion of the blowout preventer body housing the lower sealing element is of a higher working pressure than the portion of the blowout preventer body housing the upper sealing element.
 4. The method of claim 1, further comprising the upper sealing element and the lower sealing element are in different blowout preventer bodies.
 5. The method of claim 4, further comprising the upper sealing element and the lower sealing element are in separate blowout preventer bodies and blowout preventer body housing the lower sealing element is of a higher working pressure than the blowout preventer body housing the upper sealing element.
 6. The method of claim 1, further comprising the relief valve is remotely controllable.
 7. The method of claim 1, further comprising the sealing elements are the sealing element of an annular blowout preventer.
 8. The method of claim 1, further comprising the sealing elements are the rams of a ram style blowout preventer.
 9. The method of claim 1, further comprising one of the sealing elements is the sealing element of an annular blowout preventer and one of the sealing elements is the sealing element of an annular blowout preventer. 